Miniaturization is always required in the design of an RF circuit of a communication device such as a cellular phone. In recent years, the cellular phone is required to implement various functions and, to this end, it is preferable to incorporate as many components as possible in the device. However, there is a limitation in the size of the cellular phone, which makes it difficult to reduce the occupying area (mounting area) and height dimension of the RF circuit within the communication device. Thus, the components constituting the RF circuit are required to be small in terms of the occupying area and height dimension.
Under such circumstances, a thin film piezoelectric filter formed using a thin film piezoelectric resonator which is compact and capable of reducing weight has come to be utilized as a bandpass filter used in the RF circuit. The thin film piezoelectric resonator has a structure in which a piezoelectric layer made of aluminum nitride (AlN), zinc oxide (ZnO) or the like is formed on a semiconductor substrate in a sandwiched manner between upper and lower electrodes, and an oscillation space or an acoustic reflecting layer is formed immediately under the piezoelectric layer so as to prevent an elastic wave energy from propagating into the semiconductor substrate.
As described above, the thin film piezoelectric resonator can roughly divided into two types. The first one is a Film Bulk Acoustic Resonator (FBAR) having a structure in which a cavity is formed immediately under a piezoelectric resonator stack including an upper electrode, a lower electrode and a piezoelectric layer. The second one is a Surface Mounted Resonator (SMR) having a structure in which a piezoelectric resonator stack is formed on an acoustic reflecting layer obtained by alternately stacking two layers having different acoustic impedances on a substrate.
Non-Patent Documents 1 and 2 point out that the thin film piezoelectric resonators described above are subject to deterioration in terms of resonator characteristics, especially, subject to reduction in the impedance at the antiresonant frequency due to influence of conductivity of the semiconductor substrate. Further, Patent Document 1 proposes a method that forms an insulating layer on the entire upper surface of the semiconductor substrate so as to reduce the influence of the conductivity of the semiconductor substrate against the resonator characteristics.    Patent Document 1: JP-A-2003-318696    Non-Patent Document 1: “INFLUENCE OF SUBSTRATE CONDUCTIVITY ON CHARACTERISTICS OF ZnO/SiO2-DIAPHRAGM PIEZOELECTRIC RESONATORS”, Electron Letters, 1983, vol. 19, pp. 521-522    Non-Patent Document 2: “TEMPERATURE COMPENSATED HIGH COUPLING AND HIGH QUALITY FACTOR ZnO/SiO2 BULK WAVE RESONATORS ON HIGH RESISTANCE SUBSTRATES”, Proceedings of IEEE Ultrasonics Symposium 1984, pp. 405-410